Effect of counter anions and side-chain modification of conjugated polymers for organic solar cells as an interlayer: An in-depth investigation of the diverse modification of ionic functionality

Abstract

A conjugated polymer based on dimethylaminopropyl fluorene and naphthalene diimide (PFN-NDI) was synthesized to apply for organic solar cells (OSC) as the interlayer. Further, we convert the amine group to quaternary ammonium salt with various types of counter anion (CA) such as bromide and iodide, in addition to the side chain modification with hydroxyl groups. These materials generate a favorable interface dipole by the ionic functionality at the end of the side chain, while the polar hydroxyl groups increase the magnitude of interface dipole. The power conversion efficiency (PCE) was improved by incorporating polyelectrolytes as the interlayer. The PCE of the device based on PFN-NDI with acetic acid additive as the interlayer was enhanced over the device with pristine ZnO from 8.40% to 8.51%. The PCE of the device based on PFN-NDI-Br was enhanced up to 9.02%, due to the significant improvement of FF. The incorporation of extra hydroxyl groups increases the PCE up to 9.15% as a result of the synergy effect in the interface dipole moment. Furthermore, the PCE improvement was obtained by using PFN-NDI-I as the interlayer, up to 9.41%, which is mainly resulted from the Jsc enhancement. This result suggests a correlation between device performance and ionic functionality modification.